The present invention relates to an electric power assisted steering system.
Known electric power assisted steering systems include two controllers. A main controller receives signals from a vehicle speed sensor and a torsion sensor. In response to the received signals, the main controller generates an output signal to control an electric motor that provides steering assistance for turning steerable wheels of a vehicle. A secondary controller also receives signals from the vehicle speed sensor and the torsion sensor. In response to the received signals, the secondary controller determines a desired motor torque for the electric motor. The secondary controller also receives a signal from a motor current sensor that senses the actual current of the electric motor. The secondary controller compares the desired motor torque to the actual motor current and, if the difference between the desired motor torque and the actual motor current exceeds a predetermined value, the secondary controller disables the electric motor. Such a system is described in U.S. Pat. Nos. 5,259,473 and 5,271,474.
Two controllers increase both the size and the cost of the electric power assisted steering system. Additionally, since signals from the vehicle speed and the torsion sensors are input into each controller, the installation requires a separate electrical connection to each controller.
The present invention is an electric power assisted steering system for a vehicle. The electric power assisted steering system comprises a steering gear having a rack bar and a housing. The rack bar is movable linearly relative to the housing for turning steerable wheels of the vehicle. An electric motor is coupled with the rack bar of the steering gear. Energization of the electric motor causes linear movement of the rack bar relative to the housing. At least one vehicle condition sensor senses a vehicle condition and generates a signal indicative of the vehicle condition. A motor current sensor senses an actual current of the electric motor and generates a signal indicative of the actual motor current. A single integrated circuit includes a main processor and a monitoring processor. The main processor receives the at least one vehicle condition signal and, in response, controls the electric motor by controlling the actual motor current delivered to the electric motor. The monitoring processor receives the at least one vehicle condition signal and, in response, determines a desired motor torque. The monitoring processor also receives the actual motor current signal and compares the actual motor current to the desired motor torque to determine if the actual motor current is in an acceptable range and thus, determine if the main processor is properly functioning.